1. Field of the Invention
The present invention relates generally to methods and apparatus for reproducing images and alphanumeric characters, more particularly to inkjet hard copy apparatus and, more specifically to a thermal inkjet, multi-nozzle drop generator, printhead construct and its method of operation.
2. Description of Related Art
The art of inkjet hard copy technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ inkjet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Inkjet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
It has been estimated that the human visual system can distinguish ten million colors. Printing systems use a small subset of colors, yet can create acceptable reproductions of original images. Generally speaking, this is achieved by mixing the primary colors (red, blue green--additive; or cyan, magenta, yellow--subtractive) in sufficiently small quanta and exploiting tristimulus response idiosyncrasies of the human visual system. Effective use of these small quanta can be achieved in dot matrix color printing by varying the density or area fill, or both, to recreate each color or a reasonable semblance thereof in the image.
The quality of a printed image has many aspects. When the printed matter is an image, that is, a reproduction of an original image (that is to say, a photograph or graphic design rather than merely text printing), it is the goal of an imaging system is to accurately reproduce the appearance of the original. To achieve this goal, the system must accurately reproduce both the perceived colors (hues) and the perceived relative luminance ratios (tones) of the original. Human visual perception quickly adjusts to wide variations in luminance levels, from dark shadows to bright highlights. Between these extremes, perception tends toward an expectation of smooth transitions in luminance. Printing devices and similar imaging systems generally create an output that reflects light to provide a visually observable image. Exceptions such as transparencies exist, of course, but for consistency, the term reflectance will be used to denote the optical brightness of the printed output from a printing device. Generally speaking, reflectance is a ratio of the light reflected from a surface to that incident upon it. The colorants deposited upon the medium by inkjet printers are usually considered to be absorbers of particular wavelengths of light energy. This selective absorption prevents selected wavelengths of the light energy incident upon the medium from reflecting from the medium and is perceived by humans as color. Imaging systems have yet to achieve complete and faithful reproduction of the full dynamic range and perception continuity of the human visual system. While the goal is to achieve true photographic image quality reproduction, imaging systems' dynamic range printing capabilities are limited by the sensitivity and saturation level limitations inherent to the recording mechanism. The effective dynamic range can be extended somewhat by utilizing a non-linear conversion that allows some shadow and highlight detail to remain.
In inkjet technology, which uses dot matrix manipulation to form both images and alphanumeric characters, the colors and tone of a printed image are modulated by the presence or absence of drops of ink deposited on the print medium at each target picture element (known as a "pixel") generally represented as a superimposed rectangular grid overlay of the image. The medium reflectance continuity--tonal transitions within the recorded image on the medium--is especially affected by the inherent quantization effects of using ink drops and dot matrix imaging. These effects can appear as a contouring in printed images where the original image had smooth transitions. Moreover the imaging system can introduce random or systematic reflectance fluctuations (graininess--the visual recognition of individual or clusters of dots with the naked eye).
Perceived quantization effects which detract from print quality can be reduced by decreasing the density quanta at each pixel location in the imaging system and by utilizing techniques that exploit the psycho-physical characteristics of the human visual system to minimize the human perception of the quantization effects. It has been estimated that the unaided human visual system will perceive individual dots until they have been reduced to less than or equal to approximately twenty to twenty-five microns in diameter in the printed image. Therefore, undesirable quantization effects of the dot matrix printing method are reduced in the current state of the art by decreasing the size of each drop and printing at a high resolution; that is, a 1200 dots per inch ("dpi") printed image looks better to the eye than a 600 dpi image which in turn improves upon 300 dpi, etc. Additionally, undesired quantization effect can be reduced by utilizing more pen colors with varying densities of color (e.g., two cyan ink print cartridges, each containing a different dye load (the ratio of dye to solvent in the chemical composition of the ink) or containing different types of chemical colorants, dye-based or pigment-based).
To reduce quantization noise effects, print quality also can be enhanced by firing multiple drops of the same color or color formulation at each pixel resulting in more "levels" per color and reducing quantization noise. Such methods are discussed in U.S. Pat. No. 4,967,203 to Alpha N. Doan et al. for an "Interlace Printing Process", U.S. Pat. No. 4,999,646 to Jeffrey L. Trask for a "Method for Enhancing the Uniformity and Consistency of Dot Formation Produced by Color Ink Jet Printing", and U.S. Pat. No. 5,583,550 to Mark S. Hickman et al. for "Ink Drop Placement for Improved Imaging" (each assigned to the assignee of the present invention).
One can also reduce graininess in a picture by essentially low pass filtering the printed image with smoothing techniques that decrease resolution but, importantly, reduce noise. One such technique dilutes the ink (by one-fourth the original optical density by adding three parts solvent) such that the ink drop which would have been deposited on a single pixel (in, for example, a 600 dpi resolution) is spread over at least portions of adjacent pixel areas. While each drop would contain the same amount of colorant, the additional solvent causes the colorant to be distributed over a wider area. As stated, this lowers the visual noise at the cost of perceived resolution. Additionally, this technique places substantially more solvent on the printed medium resulting in an unacceptably long time to dry, consumes much more fluid consumables for printing, and slows down the speed of printing.
Large drops create large dots, or larger groups of dots that are quite visible in transition zones. Moreover, each of these methods consume ink at a rapid rate and are thus more expensive to operate. Drop volume control and multi-drop methods of inking are taught respectively by U.S. Pat. No. 4,967,208 to Winthrop D. Childers for an "Offset Nozzle Droplet Formation" and U.S. Pat. No. 5,485,180 to Ronald A. Askeland et al. for "Inking for Color-Inkjet Printers, Using Non-Integral Drop Averages, Media Varying Inking, or More Than Two Drops Per Pixel" (each assigned to the assignee of the present invention). In a multi-drop mode, the resulting dot will vary in size or in color depending on the number of drops fired at an individual pixel or superpixel and the constitution of the ink with respect to its spreading characteristics after impact on the particular medium being printed (plain paper, glossy paper, transparency, etc.). The reflectance and color of the printed image on the medium is modulated by manipulating the size and densities of drops of each color at each target pixel. The quantization effects of this mode can be reduced in the same ways as for the single-drop per pixel mode. The quantization levels can also be reduced at the same printing resolution by increasing the number of drops that can be fired at one time from nozzles in a printhead array and either adjusting the density of the ink or the size of each drop fired so as to achieve full dot density. However, simultaneously decreasing drop size and increasing the printing resolution, or increasing the number of pens and varieties of inks employed in a hard copy apparatus is very expensive, so older implementations of inkjet hard copy apparatus designed specifically for imaging art reproduction generally use multi-drop modes or multiple passes to improve color saturation.
When the size of the printed dots is modulated, the image quality is very dependent on dot placement accuracy and resolution. Misplaced dots leave unmarked pixels which appear as white dots or even bands of white lines within or between print swaths (known as "banding"). Mechanical tolerances become increasingly critical in the construction as the printhead geometries of the nozzles are reduced in order to achieve a resolution of 600 dpi or greater. Therefore, the cost of manufacture increases with the increase of the resolution design specification. Furthermore, as the number of drops fired at one time by multiplexing nozzles increases, the minimum nozzle drop volume decreases, dot placement precision requirements increase. Also the thermal efficiency of the printhead becomes low, leading to high printhead temperatures. High printhead temperatures can lead to reliability problems, including ink out-gassing, erratic drop velocities due to inconsistent bubble nucleation, and variable drop weight due to ink viscosity changes.
When the density of the printed dots is modulated as in multi-dye load ink systems, the low dye load inks require that more ink be placed on the print media, resulting in less efficient ink usage and higher risk of ink coalescence and smearing. Ink usage efficiency decreases and risk of coalescence and smearing increases with the number of drops fired at one time from the nozzles of the printhead array.
Another methodology for controlling print quality is to focus on the properties of the ink itself. When an ink drop contacts the print media, lateral diffusion ("spreading") begins, eventually ceasing as the colorant vehicle (water or some other solvent) of the ink is sufficiently spread and evaporates. For example, in U.S. Pat. No. 4,914,451 to Peter C. Morris et al., "Post-Printing Image Development of Ink-Jet Generated Transparencies", assigned to the assignee of the present invention, lateral spreading of each drop is controlled with media coatings that control latent lateral diffusion of the printed ink dots. However, this increases the cost of the print media. Lateral spreading also causes adjacent drops to bleed into each other. The ink composition itself can be constituted to reduce bleed, such as taught in U.S. Pat. No. 5,196,056 for an "Ink Jet Composition with Reduced Bleed" to Keshava A. Prasad and assigned to the assignee of the present invention. However, this may result in a formulation not suitable for the spectrum of available print media that end users may find desirous.
One apparatus for improving print quality is discussed in a very short article, Bubble Ink-Jet Technology with Improved Performance, by Enrico Manini, Olivetti, presented at IS&T's Tenth International Congress on Advances in Non-Impact Printing Technologies, Oct. 30-Nov. 4, 1994, New Orleans, La. Manini shows a concept for, "better distributing the ink on the paper, by using more, smaller droplets . . . utiliz(ing) several nozzles for each pressure chamber, so that a fine shower of ink is deposited on the paper." Sketches are provided by Manini showing two-nozzle pressure chambers, three-nozzle chambers, and four-nozzle chambers. Manini shows the deposition of multiple droplets of ink within a pixel areal dimension such that individual drops are in adjacent contact or overlapping. Manini alleges the devices abilities: to make a square elementary dot to thereby provide a 15% ink savings and faster drying time; to create better linearity in gray scaling; and to allow the use of smaller nozzles which allow higher capillary refill (meaning a faster throughput capability--generally measured in printed pages per minute, "ppm"). No working embodiment is disclosed and Manini himself admits, "The hydraulic tuning between the entrance duct and the outlet nozzles is however rather complex and requires a lot of experimentation."
Manini, however, only followed along the path of prior U.S. Pat. No. 4,621,273, filed on Dec. 16, 1982, teaching a "Print Head for Printing or Vector Plotting with a Multiplicity of Line Widths" to Dean A. Anderson and assigned to the assignee of the present invention. Anderson shows a multi-nozzle arrangement (a "primitive") for an 80-100 dpi raster/vector plotter with ink jet nozzles at selected points of a two-dimensional grid. However, while Anderson teaches a variety of useful primitive patterns (see e.g., FIGS. 1A-2B therein), the dot pattern is specifically limited by having only one nozzle in any given row or column. Selective firing is then directed depending on the plot to be created. A heavy interlacing of dots is required as demonstrated in FIGS. 4 and 5 therein.
Another problem with thermal inkjet printheads is the phenomenon known as "puddling." An ink drop exiting an orifice will tend to leave behind minute amounts of ink on the nozzle plate surface about each orifice. As these puddles grow, surface tension between the puddle and an exiting ink drop will tend to attract the tail of the drop and change its trajectory. A change in trajectory means the drop will not hit its targeted pixel center, introducing printing errors on the media. Tuning of nozzle plates is proposed in U.S. Pat. No. 4,550,326 to Ross R. Allen et al. for "Fluidic Tuning of Impulse Jet Devices Using Passive Orifices" (assigned to the assignee of the present invention).
Another problem in inkjet printing occurs at higher resolutions, for example, in multi-pass and bidirectional 300 dpi printing. Misaligned drops from different drop offsets created when the printing is accomplished by first scanning the printhead left-to-right then scanning right-to-left, cause adverse consequences such as graininess, hue shift, white spaces, and the like. This forces most printers yielding photographic color quality to always print in one direction or to interlace the scans. In either event (or when using both) the printer is forced to print at a lower printing speed. See, for example, U.S. Pat. No. 5,369,428, "Bi-directional Ink Jet Printing" to Robert C. Maze et al. Normally, binary drops are deposited in a "blackout" pattern on the grid of square pixels such that drops overlap to a degree necessary to ensure no visible white spaces occur at the four comers of the target pixel (as taught by Trask, Doan, and Hickman, supra). As mentioned, ink usage is dramatically increased by these techniques. Moreover, print media line feed error is significant compared to drop size and, without multiple-drop or overlap between pixels, white banding between swaths occurs. Thus, each of these prior art inventions are using more ink than would be required if perfectly accurate trajectories of perfectly sized ink drops could be achieved.
Therefore, until a technological breakthrough to achieve such perfection is attained, there remains a need for improvement in thermal inkjet printheads and methods of distribution of ink drops to achieve superior print quality, decreasing quantization effects, and ink usage. The goal is to reduce the required reflectance and color quantization levels of an inkjet printing system for high printing fidelity without requiring higher dot placement printing resolution while also increasing data throughput.